This invention relates generally to the radiation degradation of polytetrafluoroethylene (PTFE) to provide an improved PTFE product.
Several techniques generally make use of electron beam or cobalt sources for irradiating PTFE. U.S. Pat. No. 3,766,031 to Dillon discloses a method for radiation processing of PTFE, wherein PTFE is exposed to radiation and thereafter subjected to comminution to provide a fine particle powder. The PTFE starting material is arranged in trays and exposed to multiple doses of radiation, approximately 2 to 15 M Rad per pass, so that the temperature of the material does not rise excessively, thereby to avoid discoloration of the material and the possible generation of noxious gases. The total dose is approximately 35 to 150 M Rad. Among other things, the resulting powder is useful as a dry lubricant in paints and inks.
U.S. Pat. No. 5,576,106 to Kerbow et al. ("Kerbow") discloses a method of radiation-induced grafting of ethylenically unsaturated components such as maleic anhydride onto fluoropolymers to provide a powder that is effective as an adhesive to join dissimilar materials. Suitable fluoropolymers include copolymers of ethylene with tetrafluoroethylene. Kerbow discloses that the average particle size of the base fluoropolymer is 50-500 .mu.m. The fluoropolymer and the grafting compound are mixed or otherwise brought into proximity in the presence of ionizing radiation doses in the range of about 2-6 M Rad. Kerbow discloses that the grafted powder can be applied to a surface from a dispersion or can be blended with a liquid. Kerbow does not disclose how to provide a friable polymer product having smaller particle sizes, nor does Kerbow teach specific handling techniques to prevent the formation of agglomerations or fibers upon subjecting the polymer products to grinding or shearing.
U.S. Pat. No. 4,029,870 to Brown et al. ("Brown '870") and U.S. Pat. No. 4,036,718 to Brown et al. ("Brown '718") each disclose processes for providing dispersions of PTFE in organic solvents. The processes entail irradiating PTFE and subsequently adding the resulting product to the organic solvent. The combination is then subjected to high shear mixing to break PTFE agglomerations down to sub-micron size particles and to thereby provide the dispersion. Brown '718 and Brown '870 further teach that this dispersion may be introduced into oils and greases, and is useful in formulating PTFE coatings. U.S. Pat. No. 4,052,278 to Brown et al. ("Brown '278") further discloses a process in which PTFE is first irradiated, then dry milled, and then dispersed in an aqueous or organic solvent. Brown '278 teaches that this process facilitates dispersion as compared to simply blending PTFE with a dispersing medium during a high shear mixing step.
U.S. Pat. No. 4,777,192 to Neuberg et al. discloses an apparatus and method for radiation degradation of PTFE, wherein radiation is supplied to a zone of a vessel, and agitation is provided to move portions of the PTFE material into and out of the irradiated zone. Provision is made for cooling the PTFE, including a cooling jacket or an addition of water to the reaction vessel. Neuberg et al. further teaches that an addition of water lowers the oil absorption of the resulting powder. An addition of air or oxygen to the reaction mixture is also disclosed.
British Patent 1,516,648 to Kholodov et al. discloses a method for regenerating PTFE wherein previously degraded PTFE is irradiated in the presence of fluorinated monomers. The process thereby causes a grafting of the monomers onto the PTFE. Kholodov further discloses a use of cooling water circulating through a jacket on the reactor vessel, and an addition of water directly to the PTFE and fluorinated monomers. The Kholodov reference teaches radiation regeneration, rather than radiation degradation. Accordingly, the effect of practicing the Kholodov method is to increase, rather than decrease the molecular weight of the PTFE. The reference also teaches that oils should be removed from the PTFE prior to irradiation.
U.S. Pat. No. 3,838,030 to Kigiya et al. discloses a process for preparing PTFE resin wax, whereby PTFE is degraded by an ionizing radiation in the presence of a gaseous mixture of air or oxygen with hydrogen and/or the vapor of halogenated methane. Prior to irradiation, the material is held at a temperature of 350.degree. C. (662.degree. F.).
U.S. Pat. No. 4,220,511 to Derbyshire discloses a method for providing a grindable PTFE material in which PTFE is irradiated in the presence of air or oxygen on a conveyer with concurrent or subsequent heating to a temperature of from 150.degree. F. to 600.degree. F. by an air-circulating oven. U.S. Pat. No. 2,858,442 to Dewey discloses an apparatus for irradiating: (1) a flexible sheet material; (2) a multiplicity of thin sheets; and (3) a filamentary material. Dewey also teaches that fluids may be irradiated by conveying them through an electron beam in a direction parallel to the direction of the electron flow.
The prior art methods of radiation degradation of PTFE do not provide friable PTFE products having micron and sub-micron sized particles that are flowable and/or easily dispersed in, for example, paints, inks, plastics, adhesives, motor oil and oil additives, and components thereof. Milling methods have limited ability to impart friability to such PTFE, or to provide fine, flowable powders that do not agglomerate. In addition, although the PTFE products of such methods can be dispersed in certain organic solvents with the aid of surfactants, a low surface energy often presents wetting limitations. Accordingly, a need exists for a simple technique for providing friable PTFE products having micron and sub-micron sized particles which exhibit superior flowability and/or dispersion characteristics in a variety of media.